Inhibition of p53 function, either through mutation or inhibition by viral transforming proteins, correlates strongly with the oncogenic potential of the cell. In the following report, we describe a unique mechanism of p53 inactivation that involves the interaction of p53 with the p65 subunit of NF-kB. The inactive p53 complex is induced in HTLV-I-transformed and ATL leukemic cells. It will be of interest to determine if this mechanism of p53 inhibition occurs in other human cancers. Only a small percentage of human T-lymphotropic virus type-I (HTLV-I)-transformed cells carry p53 mutations, and mutated p53 genes have been found in only one-fourth of ATL cases. We demonstrated that the wild-type p53 is stabilized and transcriptionally inactive in HTLV-transformed cells. The viral transcriptional activator Tax plays a role in both the stabilization and inactivation of p53 through a mechanism involving the first 52 amino acids of p53. We further demonstrated that p53 is hyperphosphorylated at serines 15 and 392 in HTLV-I-transformed cells and that phosphorylation of p53 at these specific residues inactivates p53 by blocking its interaction with basal transcription factors. In T-lymphocytes, Tax-induced p53 inactivation is dependent upon NF-kB activation. Analysis of Tax mutants demonstrated that Tax inactivation of p53 function correlates with the ability of Tax to induce NF-kB. Further, the p65 subunit of NF-kB is critical for Tax-induced p53 inactivation using wild-type and p65 knockout MEFs. While Tax can inactivate endogenous p53 function in wild-type MEFs, it fails to inactivate p53 function in p65 knockout MEFs. Tax-induced p53 inactivation can be restored by expression of p65 in the knockout MEFs. These studies provide evidence that the divergent NF-kB proliferative and p53 cell cycle arrest pathways may be cross-regulated at several levels which include post-translational modification of p53. Using MEF knockout cells and antisense oliogonucleotides to inhibit expression of NF-kB family members, we found that the p65 subunit of NF-kB is uniquely involved in p53 inhibition. Coimmunprecipitation assays demonstrate an interaction between p65 and p53 in HTLV-I and Tax-expressing cells. Further, in transient transfection assays, we demonstrate that Tax induces the interaction between p65 and p53 and that phosphorylation of p53 at serines 15 and 392 is critical for complex formation. Using a series of NF-kB mutants, we find that Tax-mediated p53 inhibition correlates with p65 and p53 interaction. Using chromatin immunoprecipitation assays we have determined that in HTLV-I-transformed cells, p53 and p65 form a complex on the inactive MDM2 promoter. Consistent with reduced transcription activity, TFIID binding is not observed. These studies identify a unique mechanism for p53 regulation by the p65/RelA subunit of NF-kB. Checkpoint kinase 1 (Chk1) mediates diverse cellular responses to genotoxic stress, regulating the network of genome-surveillance pathways that coordinate cell cycle progression with DNA repair. Chk1 is essential for mammalian development and viability, and has been shown to be important for both S and G2 checkpoints. We now present evidence that the HTLV-1 Tax protein interacts directly with Chk1 and impairs its kinase activities in vitro and in vivo. The direct and physical interaction of Chk1 and Tax was observed in HTLV-1-infected T cells (C81, HuT 102 and MT-2) and transfected fibroblasts (293T) by coimmunoprecipitation and by in vitro GST pull-down assays. Interestingly, Tax inhibited the kinase activity of Chk1 protein in in vitro and in vivo kinase assays. Consistent with these results, Tax inhibited the phosphorylation dependent degradation of Cdc25A and G2 arrest in response to gamma-irradiation (IR) in a dose-dependent manner in vivo. The G2 arrest did not require Chk2 or p53. These studies provide the first example of a viral transforming protein targeting Chk1 and provide important insight into checkpoint pathway regulation. Ovarian cancer is the most lethal type of gynaecological cancer in the Western world. The high case fatality rate is due to in part to that fact that most ovarian cancer patients present with advanced stage disease which is essentially incurable. In order to obtain a whole genome assessment of aberrant gene expression patterns in ovarian cancer, we used oligonucleotide microarrays comprising over 40,000 features to profile 37 advanced stage papillary serous primary carcinomas. We identified 1191 genes that were significantly (p<0.001) differentially regulated between the ovarian cancer specimens and normal ovarian surface epithelium. The microarray data were validated using real time RT-PCR on 14 randomly selected differentially regulated genes. This list contained genes involved in various biologic processes, including cell growth, differentiation, adhesion, apoptosis and migration. In addition, numerous genes whose function remains to be elucidated were also identified. The microarray data were imported into PathwayAssist software to identify major signaling pathways involved in ovarian cancer tumorigenesis. Based on our expression data, a signaling pathway associated with tumor cell migration, spread and invasion was identified as being activated in advanced ovarian cancer. The data generated in this study represent a comprehensive list of genes aberrantly expressed in serous papillary ovarian adenocarcinoma and may be useful for the identification of potentially new and novel markers and therapeutic targets for ovarian cancer.